Method of preparing fluorescent materials



December 30, 1936.

Patented Aug. 12, 1941 UNITED STATES PATENT OFFICE I .zasz soov v METHODOF PkIIAPTMG FLUORESCENT Gorton a. Fonda, Schenectady, N. r., assig'norto General Electric Company, a corporation of New York No Drawing.Application December I, 1939,

Serial lilo. soaoos scum. (01. 250-41) The present invention relates toluminescent materials characterized by phosphorescence and fluorescence(herein termed a phosphor) or. only.-

improved fluorescence as the case may be, and

consists in an improved method of preparing such materials. Thisapplication is a continuation-inpart of my application Serial No,118,323, filed As a consequence of my invention luminescent materialsmay'be prepared which not only are efllcient as light emitters, byfluorescence (that is,

giving out light of a wave length diflering from the wave length ofradiations impinging on the material), but which alsoe'xhibit markedphosphorescence (that is, giving out light during an appreciableinterval after the exciting radiation has ceased). One of thecharacteristics of my improved materials is greater retention offineness of the particles during heat treatment, so that the firedproduct may be used without the depreciating effect which extensiveregrinding has on a fluorescent powder.

In one of its aspects my invention is characterized by improvement inthe facility of preparation and decrease of time required for'thepreparation of luminescent materials.

'It'has been suggested heretofore to prepare fluorescent material bymixing the powdered ingradients and subjecting the mixture tothermosynthesis, that is, to reaction produced by firing at hightemperature, but the resulting product is apt to be so coarse-grained asto require regrinding. The effectiveness of the product for fluorescentpurposes is reduced by grinding after the final firing step.

In accordance with my present invention the source materials of a chosenphosphor, that is, materials iromwhich the phosphor may be made,

are caused to assume the state of a colloidal gel,

which is converted by appropriate treatment to the state of afluorescent powder. For example, in the fabrication of a siliceousfluorescent substance, a solution of a chosen metallic compound is mixedwith a'compound of silica, a suitable activator also being added. Themixture is convertecl to a gel,the ingredients being'intimately mixedwithout the likelihood of becoming segregated when the gel is convertedto a solid product. The composition finally is heat-treated until itassumes the desired fluorescent condition.

7 My invention will be more fully understood by a consideration of thefollowing specific examples. the novel features of the invention beingset forth in the appended claims.

-' manganese) is added to a suitable compound containing silicon as 'aconstituent, for example. an organic compound such as ethyl silicate oran inorganic compound suchthe proportions desired. It maximumphosphorescence is desired, the proportions-of the ingredients should beequimolecular, in terms of one mol 19 of zinc oxide and one moi ofsilica, the final product probably being an agglomerate oi ZmSiO4 and310:. If phosphorescence is not desired in the product. the proportionsmay be chosen to yield two mols of zinc oxide and one moi of silica,and, it indeed, other proportions may be used. If the chosen proportionsare two mols of ZnO and one moi of 810:, then the zinc silicate ZnzSiO4is iormed. This product has only slight phosphorescence and has the samefluorescence as the so equimolecular ratio.

The zinc nitrate, or other zinc compound, preferabiy is dissolved inethyl alcohol, or other suitable solvent. I prefer to use an alcoholicsolution containing about 25 per cent water. For ex- 25 ample, for apreparation having the proportions of one molpof zinc oxide to one molof silica and containing 0.4 per cent of manganese, a clear solution ofethyl silicate is prepared by adding, while stirring, 100 cc.-of ethylsilicate to a mix- 30 ture of 10.7 cc. water, 14.2 cc. ethyl alcohol,and

0.1 cc. concentrated hydrochloric acid. A solution of zinc nitrate isadded. which may be prepared by dissolving 134 grams of hexahydratedzinc nitratecrystals in a mixture of 300 cc. ethyl 35 alcohol and 90 cc.of water. A small amount of manganese nitrate solution or manganesecarbonate (for'example, the equivalent of 0.25 gm. iurnish the desiredamount of the activator.

40 In place of ethyl silicate, I may use as the acid constituent of thephosphor silicates con-' taining organic groups other than the ethylgroup, for example, an alkyl silicate, such as the methyl, propyl, oramyl silicate; or I may use in- .hl'ganic compounds. such aspotassiumsilicate or faiiicon tetrachloride. In place of the zinc nitrate, thebase constituent, other soluble zinc salts ,tnay be used. such as theacetate or chloride or sulphste of zinc, orinsoluble materials may betil/ used, such as zinc oxide or zinc carbonate. Sim- "iiarly, salts ofother metals may be used to partly or wholly-replace zinc, for example,magnesium,

. cadmium, or beryllium.

The quantity of manganese, or other suitable In the preparation of. azinc silicate phosphor, 5c activator, is chosen to leave in general lessthan about 2 per cent and preferably about 0.4 cent oi metal constituentin the final product.

The liquid mixture is heated in the open to boiling, stirringsufllciently to avoid "bumping," resulting in the evaporation of enoughvolatile material to form a semisolid, colloidal siliceous gel, theformation of which is promoted by the presence of the hydrochloric acid.when the residue becomes a crumbly or easily friable solid, it iscrushed and heated to a temperature of about 110 C., until it no longercakes when the tem-- perature may be slowly raised to 130 or 140 0.,resulting in elimination of water and other volatile products, thenature of which depends on the character of the material employed. Theproduct is reground, conveniently in a ball mill per for a period ofseveral hours and denitrated by reheating up to a final temperature of350 C. After a short period of further grinding, the product finally isfired at a temperature or about 1000 to 1300 C. until the desiredfluorescent properties are developed. Care should be exercised to avoidsintering or coarsening of particle size. Heating for hours at about1100 C. is satisfactory. Heretofore, when the ingredients were mixed aspowders, a much longer flring period, ordinarily about three times aslong, was required, with a corresponding greater coarsening.

When potassium silicate is used as the source of silica, the gel may beprepared by precipitating the silicate with hydrochloric acid. -Whensilicon tetrachloride is used it is caused to react with water to formthe gel.

The resulting phosphor is applied commonly as a thin coating on theinterior surface of gaseous discharge lamps, such lamps so coated aredescribed, for example, in Riittenauer U. S. Patent 2,030,402 and FritzeU. S. Patent 2,030,440,

both patented February 11, 1936. When the pro-' portions are chosen asabove indicated to produce phosphorescent as well as fluorescentproperties, then the light output ofa lamp provided with such phosphoris steadied.

When, as heretofore themractice, a mixture of chosen oxides (forexample, zinc oxide and silica) is fired to develop fluorescence, theoxides become progressively coarser, requiring extended grinding toreduce the coarse product to sumciently fine subdivision for use influorescent lamps. Such grinding'is undesirable as it resultsfluorescence in chosen proportions in a liquid solvent therefor, heatingto form a gel, heating further at a temperature sufllciently high toremove volatile constituents from the product, comminuting said product,and firing within a range of about 1000 to 1300 C. to producefluorescent properties.

3. The method of preparing a fluorescent silicate which comprisesiorming a mixture of colloidal gel of silica formed from alkyl organicsilicate, a water soluble salt of a metal which will react with saidsilica to form said fluorescent silicate, and a water soluble activatorof fluorescent properties, heating to remove volatile constituents fromthe mixture, comminuting the product, heating at a higher temperature todrive oil the acid constituent of said metal salt and firing at anelevated temperature to develop fluorescent properties.

4. The method of preparing a fluorescent silicate which comprisesforming a mixture of colloidal gel of silica formed from alkyl organicsilicate, a water soluble salt of a metal which will react with'saidsilica to form said fluorescent silicate, a solvent for said metal salt,and

- 9. water soluble activator of fluorescent properin a serious reductionin the intensity of fluorescence. When the source constitutes for aphosphor are caused to assume the state of a gel, which is driedandheat-treated, then the product does not become coarsened so seriouslyand requires either no regrinding or very little regrinding with anaccompanying improvement in effectiveness.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of preparing a fluorescent silicate which comprisesforming a mixture of colloidal gel of silica formed from alkyl organicsilicate, a metal compound which will react with said silica to formsaid fluorescent silicate, and an activator of fluorescent properties,heating to remove volatile constituents from the mixture, comminutingthe product and firing at an elevated temperature to develop fluorescentproperties.

2. The method' of preparing a fluorescent silicate which comprisesmixing an alkyl organic silicate, a metal compound which will react withsilica gel derived from said silicate to form said fluorescent silicate,and manganese activator oi ties, heating to remove volatile constituentsfrom the mixture, comminuting the product, heating at a highertemperature to drive off the acid constituent of said metal salt'andfiring at an elevated temperature to develop fluorescent properties.

5. The method of preparing a fluorescent silicate which comprisesforming a mixture of colloidal gel of silicate formed from alkyl organicsilicate, a water soluble salt of a metal which will react with saidsilicate to form said fluorescent silicate, a solvent for said metalsalt, and a water soluble activator of fluorescent properties, heatingto convert said mixture to a solid, comminuting said solid, heatingfurther to remove volatile constituents, again comminuting the product,heating at a higher temperature to ve off the acid constituent of saidmetal salt and firing at an elevated temperature to develop fluorescentproperties.

6. The method of preparing fluorescent zinc silicate which comprisesmixing an alkyl organic silicate, a water soluble zinc salt and a watersoluble manganese activator of fluorescence in chosen proportions in aliquid solvent therefor, heating to form a gel, heating further at atemperature sufliciently high to remove volatile constituents from theproduct, comminuting said product, heating at a higher temperature todrive oif the acid constituent of said zinc salt and firing within arange of about t11000 to 1300 C. to produce fluorescent proper- 7. Themethod of preparing a fluorescent sillcate which comprises heating inadmixture with one another an alkyl organic silicate, a water solublesalt of a metal which will react with silica gel derived from saidsilicate to form said fluorescent silicate, a water soluble manganeseactivator of fluorescence and a liquid solvent therefor to form a gel,heating further to remove volatile constituents product, comminuting theproduct, heating at a higher temperature to drive oil. the acidconstituent of said metal salt and ultimately firing the resultingproduct to a temperature within the range of about 1000 to 1300 C. todevelop fluorescent properties.

8. The method of preparing fluorescent zinc and to form a solid silicatewhich comprises mixing an aqueous ethyl alcohol, a silicate of an alkylbase, a soluble zinc compound and a soluble manganese activator offluorescence, heating to remove vola tile material until a gel isformed, heating further to convert said gel to the solidstate,-comminuting said solid, heating to a temperature of about 130 to140 C., thereby further eliminating volatile material, recomminutin gthe product, reheating to about 350 (3., again recomminuting and finallyfiring to a temperature within.

the range of about 1000 to 1300 C.

p 9. In the process of forming a zinc silicate phosphor the steps whichcomprise producing a mixture consisting of ethyl silicate and zinc ni-GORTON R. FONDA.

